This invention relates to the art of processing video signals and, more particularly, to improvements in filtering video signals.
Filters are known in the art and may be employed for separating the luminance and chrominance components of a composite NTSC color television signal. Two types of filters are employed including a bandpass filter and an adaptive comb filter. A bandpass filter, for example, may operate to receive the composite television signal and operate to pass a band of frequencies, centered about the chrominance frequency of 3.58 MHz, so as to pass the chrominance components and some high frequency luminance components. This bandpass filter signal may be considered as the separated out chrominance component and the luminance component can then be obtained by subtracting the chrominance component from the composite television signal. However, this bandpass filtered chrominance component, sometimes referred to as the filtered chroma, does include some high frequency luminance components. Adaptive comb filters are known which operate in certain conditions to provide a chrominance component relatively free of luminance components.
In a typical adaptive comb filter, the incoming video signal is separated by delay lines into line components representative of three successive television scan lines. These are typically referred to as the main or middle (M) line component and two vertically adjacent line components referred to as the top (T) and bottom (B) components. The middle (M) component and the top (T) component are respectively delayed by one and two horizontal lines relative to the bottom (B) component. The chrominance signal reverses in phase from line-to-line. When the three components are combined in the amplitude ratios of T/2+B/2-M, then, the luminance is cancelled out to thereby derive the negative of the chrominance component at twice its value, i.e., -2C. This derived chrominance component is sometimes referred to as the combed chroma, which is obtained from the adaptive comb filter, as opposed to the filtered chroma. The three line components B, T, and M may also be combined in such a manner that the chrominance cancels out leaving only the derived luminance component.
The separation and cancellation technique described above assumes there are no changes or transitions in picture detail from top to bottom through three successive lines. If substantial vertical transitions do take place, then smearing of the picture occurs in the vertical direction, presenting a loss in vertical picture detail. In such case, the comb filter is made inoperative and the chrominance output is the filtered chroma from a bandpass filter.
The prior art includes transition detectors which serve to determine whether there is a sufficient vertical picture detail or vertical transition to automatically select the combed chroma or the filtered chroma as the chrominance output signal. Such transition detectors are disclosed, for example, in the U.S. Pat. No. 4,050,084, to Rossi, and U.S. Pat. No. 4,072,984 to Kaiser. Kaiser's transition detector makes the selection of combed chroma or bandpass filtered chroma on the basis of vertical picture detail or vertical transition only. Horizontal transition is not considered. Rossi's transition detector makes the selection based on vertical transitions between the middle (M) line and the top (T) line and between the middle (M) line and the bottom (B) line along with a horizontal transition between an undelayed middle line pixel (M) and a delayed middle line pixel (M'). These transitions are individually compared against a reference and based on the comparisons, the detector selects either a combed chroma or a bandpass filtered chroma to serve as the output chrominance signal.
In the presence of vertical transitions, the comb filter algorithm begins to breakdown, in that the luminance signal does not cancel out, and chroma transitions are smeared over the three lines, reducing vertical resolution. It has been determined that bandpass filtering in the presence of horizontal transitions causes horizontal smearing, since high frequency luminance components are inverted along with the chroma. Low frequency luminance components are rejected by the bandpass filter, and so do not impair the output signal.
It is seen, then, that there is a trade-off as to whether the output chrominance signal should be obtained by bandpass filtering only, or by a comb filter. Thus, if there is more horizontal detail, then combing will preserve the horizontal detail with little loss of vertical detail. If there is more vertical detail, the vertical detail may be preserved by bandpass filtering. By comparing vertical transitions against horizontal transitions, a determination can be made as to which process, combing or bandpass filtering, will best preserve picture detail.